This invention relates to a safety device for use with unvented, low power, gas burning water heaters and more particularly to a device which limits the duty cycle of the burner used with such a heater.
In certain areas, instantaneous gas water heaters not exceeding 125 millitherms per minute need not be connected to smoke ducts, provided that certain precautions required by law are strictly observed. These precautions typically require that the heater be installed in a room of at least a certain size, having adequate air vents above and below the heater. Under these circumstances, the combustion products released into the room's air are gradually evacuated through this normal room ventilation. Accidents can sometimes occur, however, if the rate at which air is renewed is too low. The risk of poisoning by combustion products is heightened, moreover, in the case of prolonged use of the water heater.
One proposed solution to this problem involves keeping a check on the air in the room in which the water heater is installed. A safety cut-off is typically actuated whenever the level of toxic gases becomes too high. While such systems are relatively uncomplicated and reliable when dealing with a well-known gas under constant conditions of use, e.g., a natural gas of fixed composition at a given pressure, they are difficult to adapt for use with so called "all gas" heaters that can use different fuels at various pressures.
It is therefore useful, since the toxicity of the air usually reaches a dangerous level only after a prolonged period of use of the water heater, to utilize a safety device whose operation is linked to the period of use, the heater's safety cut-off being activated when the heater has been used for too long of a time which might lead to a dangerous level of poisonous gas in the room.
Among the several objects of the present invention may be noted the provision of a safety device for a low power, unvented gas water heater; the provision of such a device which limits the duty cycle of such a heater; and the provision of such a device which is of relatively simple and inexpensive construction.